The present invention pertains to a type of apparatus or system typically referred to as a "tensioner" and is particularly well adapted to those tensioners which are used offshore in connection with marine riser pipe structures. In connection with the drilling and production of offshore oil and gas wells, various operations are typically conducted from a support structure, such as a drill ship or semi-submersible platform, which rises and falls with the wave action. One or more wellheads are located on the floor of the body of water generally beneath this structure. For each such wellhead, a package of equipment, e.g. a stack of blowout preventers, is run into place on a string of relatively large diameter pipe known as riser pipe. After this running in operation, the riser pipe is left in place so that it extends upwardly from the wellhead area to a point above the surface of the water and near the support platform whereby it may serve as a rough guide for other strings of apparatus which must, from time to time, be lowered to the wellhead and/or into the well.
The riser pipe must be supported with respect to the platform or other support structure for several reasons including the prevention of collapse of the riser pipe under its own weight as well as the prevention of excessive swaying motion of the riser pipe in the water. Accordingly, it is customary to support the riser pipe with respect to the platform by placing it under considerable tension, the magnitude of the tension load typically exceeding the weight of the riser pipe.
Maintaining the aforementioned tension load at a given value, or at least within a given range of values, is rendered difficult because of the heaving of the platform or other support structure due to wave action, etc. Thus, the tension cannot be applied by a static or fixed system. U.S. Pat. No. 3,314,657 to Prud'homme et al discloses a typical tensioner system. To the extent that this patent is helpful in understanding the type of apparatus to which the present invention is applied, it is hereby expressly incorporated herein by reference.
Briefly, a typical tensioner, such as that disclosed in the Prud'homme et al patent, includes a piston and cylinder arrangement interconnected between the offshore platform and the riser pipe in such a way that relative movements of the platform and riser pipe tend to cause corresponding relative reciprocation of the piston and cylinder, and conversely, reciprocations of the piston and cylinder tend to cause or at least permit relative movement of the two interconnected offshore structures.
A high pressure fluid is applied against the piston in one end of the cylinder, and it is the force of this pressurized fluid which ultimately supports the riser pipe with respect to the platform and applies the desired tension. The high pressure fluid, or at least a portion thereof, is compressible. More specifically, the body of high pressure fluid may be comprised entirely of a gas, or it may be a suitable liquid, such as oil, backed by a volume of pressurized gas. This permits reciprocation of the piston and cylinder, so as to accommodate relative movement of the platform and riser pipe, while still maintaining the tension load on the riser pipe within a given range of values.
In order to prevent the piston from slamming or jolting action when it reciprocates away from the high pressure end of the cylinder, a lower pressure balancing fluid is admitted into the opposite end of the cylinder. This low pressure fluid may flow into and out of the cylinder to permit the necessary reciprocations of the piston, but its flow rate is controlled by a throttle or the like which slows the piston speed, at least near the end of its stroke, so as to avoid undesirable slamming or jolting during normal operational reciprocations.
One problem which has arisen in connection with the type of tensioner described above occurs when the riser pipe, or a portion thereof, is suddenly disconnected from the wellhead apparatus while the tensioners are still engaged. Such disconnections may occur due to accidents, e.g. failures of the riser pipe itself or some related wellhead apparatus, or through the operation of emergency disconnect systems which are used to disconnect the riser pipe, e.g. in the event of severe weather. Such occurrences cannot be completely avoided, and it is not practicable to disengage the tensioners before disconnection of the riser pipe. In tensioning the riser pipe, the system applies a large vertically upward force. If the riser pipe is disconnected, the tensioners will continue to apply this force, but the riser pipe, no longer anchored to the wellhead, will be raised upwardly toward the platform. The riser pipe will, in effect, be suddenly jerked upwardly at a relatively high rate of speed. The heavy riser pipe can gather such momentum that it will ultimately collide with the platform and may cause serious damage or injury. The particulars of such collision will vary depending upon the location of the support platform with respect to the wave crests and troughs at the time of disconnection of the riser pipe. However, it can be shown that disconnection, at whatever point, can result in a dangerous and expensive collision at virtually any point. As we continue drilling at greater and greater depths, riser pipe structures become heavier, and the potential magnitude of such collisions increases.
The throttling means disclosed in the aforementioned patent to Prud'homme et al merely suffices to cushion the advancing piston toward the end of its stroke during normal operational reciprocation, but will not suffice to control the extremely high speeds and forces which prevail when the heavy and highly tensioned riser pipe is suddenly disconnected from the wellhead.
U.S. Pat. No. 4,351,261 to Shanks discloses a system intended to deal with such emergency conditions, but this system is not entirely satisfactory for several reasons. In the first place, the system of the Shanks patent operates to brake the upward movement of the piston, and thus that of the disconnected riser pipe, by applying a high pressure fluid to what is normally the low pressure end of the tensioner cylinder, or otherwise adjusting or replacing the sources of fluid pressure so as to tend to equalize the pressure on both sides of the piston. This system is accordingly unduly complicated and expensive. Furthermore, the nature of the system is such that it tends to hold the piston in a centered position in the cylinder, whereas in many instances, it would be desirable to permit the piston to advance to the extreme end of its stroke so as to raise the riser pipe structure as high as possible.